Electronic integrated circuits are routinely produced by photolithography using a photoresist coated onto a semi-conductor wafer. In such processes the wafer is coated with a layer of the photoresist which is then subjected to a soft bake (e.g. at about 90.degree. to 100.degree. C.) to remove the solvent. The prepared photoresist is then exposed through an imaging negative to form the desired circuit pattern on the surface of the photoresist and then developed to produce a replica of the circuit pattern in the developed resist.
Imaging of positive photoresists is carried out by various optical systems including both contact printers and projection equipment. The demand for greater miniaturisation of circuits increases the problems of resolving finer and finer image lines in photoresists. The maximum possible resolution of fine detail obtainable depends both on the quality of the optical system and on the ability of the photoresist materials to reproduce, in sharp relief, such fine detail. In the present state of the art, the limiting factor is the ability of the developed photoresists to reproduce the desired pattern. This arises because of the difficulty of maintaining sufficient contrast between fine lines in the pattern for a sharp image to be reproduced. As contrast falls with decreasing line width, discrimination of darker areas in a pattern from lighter areas becomes increasingly difficult. On development of the resulting exposed photoresists, wall angles of the pattern depart more and more from the vertical, thus representing poorer definition. If a satisfactory way can be provided for enhancing the contrast, finer detail can be reproduced more sharply without any requirement for corresponding improvements in the optical system.
Contrast enhancement can be achieved by coating the incident surface of the photoresist with a layer of material which is opaque but is bleachable by the radiation employed to expose the photoresist. As a consequence, an in situ mask is formed on the surface of the photoresist, the pattern of which corresponds to that of the imaging negative and this mask has the effect of amplifying the contrast between light and dark areas.
For example, U.K. Patent Specification No. 2131190 (General Electric) describes a process for producing integrated circuits which utilise such a contrast enhancement technique. However, the contrast enhancement layer in the General Electric process cannot be removed using the photoresist developer but necessitates a separate removal step. Furthermore, the photochromic compounds described in the above General Electric patent specification have practical disadvantages, such as sensitivity to water and a tendency to undergo fatigue rections when stored in solution. Also, the polymer solutions of photochromic compounds employed to form the contrast enhancement layer develop a tacky surface which prevents their use in contact lithography.
U.K. patent specification Nos. 513029 (Kalle), describes a diazotype reflex copying process in which two, similar light-sensitised transparent sheets are placed in contact and together exposed to a light pattern of an original. However, there is no disclosure of any use of the copying process to prepare photoresists.
U.K. patent specification No. 1294105 (Kalle), is concerned with the production of planographic printing formes. A planographic forme is produced by coating an aluminium plate with a first light sensitive layer and applying a second layer or film to the first layer. The second layer or film contains a light-absorbing chemical which captures more of the incident light on image-wise exposure of the coated plate than would the light sensitive layer alone.